Abstract：Based on energy conservation law, the efficiency at the resonant point of LLC resonant converter was calculated, and accordingly the resonant frequency and the magnetizing inductor were optimized. Then the effecting curves of the resonant inductor on the minimum switching frequency and the maximum output voltage were analyzed, and accordingly the resonant inductor was optimized. Simultaneously, to increase the design accuracy of resonant parameters, the leakage inductor of transformer was calculated based on energy method and considered in the design process of resonant inductor. To verify the proposed design method, a 3.3kW LLC resonant converter prototype applied for on-board charger was developed, where the output voltage ranged from 230V to 430V, the efficiency was higher than 95.9% in the full output voltage range, and the peak efficiency was higher than 97.5%. The experimental results verify that the proposed design method improves the efficiency of the converter in a wide output voltage range.